This invention relates to electrical circuits for supplying positive and negative air ions, and more particularly to embodiments of air ionizers that operate on alternating current (AC) and include direct current (DC) biasing for promoting substantially zero residual electrostatic charges on target objects.
Air ionizing apparatus that produces both positive and negative air ions can be used to reduce electrostatic charges on various objects such as semiconductor wafers and die during fabrication processes. However, reducing the level of electrostatic charges to the grounded level can be difficult because negative ions are more readily produced and transported through air from an ion generator to the object than positive ions.
Conventional AC air ionizers differ from DC or pulse-type ionizers because all emitter points exhibit the same electrostatic field gradient on applied AC voltage at the same time. There are thus no bipolar potentials on spaced emitter points at any given time as with DC air ionizers, so charge neutralization by AC air ionizers over the area of an object tends to be more uniform. However, the swings in voltages attributable to residual charges on surfaces of objects tend to fluctuate with the frequency at which the AC ion generator produces air ions. Controlling high ionizing voltages, for example, via feedback circuitry to diminish the fluctuations, is generally difficult so lower voltages are used and a reference electrode is disposed adjacent each emitter point to develop the necessary electric field gradient sufficient to produce corona. Certain known AC ionizers apply opposite polarities of the AC voltages to one or more pairs of space emitter points to diminish the AC voltage swings on the target object. Other known AC ionizers rely upon such waveform controls as amplitude or pulse-width or phase modulations to achieve ion balance and reduce voltage variations on the target object.
In accordance with the illustrated embodiments of the present invention, a reference electrode receives a DC bias voltage as an offsetting potential to alter the mix of positive and negative generated ions. A negative bias voltage is generally required for an isolated system, and for a positive grounded system, but the bias voltage level (and polarity) may have to vary in response to such operating conditions as the ion-generating characteristics of emitter points, and the like, in order to achieve near ground or reference level charge neutralization of a target object.